This invention generally relates to endoprosthesis devices, most often referred to as stents, and more particularly pertains to the radiopaque marking of such devices.
Stents are useful in the treatment of atherosclerotic stenosis in blood vessels and are generally tubular shaped devices which function to hold open a segment of a blood vessel or other anatomical lumen. They are particularly suitable for use in supporting and holding back a dissected arterial lining which could otherwise occlude the fluid passageway therethrough.
In order to accomplish precise placement of stents, various means are employed to identify the position of the stent within a blood vessel. One means frequently described for accomplishing precise placement of a stent is the attachment of radiopaque markers to the stent so that through the use of fluoroscopy, the position of the stent within a blood vessel can be identified. Once the stent with its radiopaque markers has been implanted, subsequent checkups of the treated segment are easily performed since the markers remain visible under fluoroscopic illumination.
Conventional radiopaque markers, however, have a number of limitations. Upon attachment to a stent, conventional radiopaque markers may project from the surface of the stent, thereby comprising a departure from the ideal profile of the stent. Such conventional radiopaque markers protrude from the walls of the stent and depending upon their location upon the stent, may either project inwardly to disrupt blood flow or outwardly to traumatize the walls of the blood vessel. In addition, conventional radiopaque markers have the disadvantage in that their attachment to the stent can be tedious and imprecise. Moreover, the configuration of many heretofore known markers fails to provide a precise indication of the location and position of the stent. Finally, the galvanic corrosion that might result from the contact of two disparate metals, i.e., the metal used in the construction of the stent and the radiopaque metal of the marker could eventually cause the marker to become separated from the stent which could be problematic should the marker embolize downstream and occlude the artery.
Other conventional radiopaque markers restrict the expansion capabilities of an expandable stent by adding rigidity to the stent in areas designated for stent deformation. Still other conventional stents utilize material, such as tantalum, that is effective for use in identifying the location of a stent within a vessel, but fluoroscopically illuminates so brightly so as to obscure proper visibility of the arterial lesion, thereby impairing the ability to repair the lesion. Finally, conventional radiopaque markers do not generally, under fluoroscopy, provide the operator with means to accurately access stent length and diameter.
Stents have also been previously marked by plating selected portions thereof with a radiopaque material. An advantageously selected pattern of plated areas would theoretically allow the position, length and diameter of the stent to be discerned. However, due to the minimal thickness of the plating, only an extremely faint fluoroscopic image can be generated which may ultimately limit its utility.
To overcome the problems and limitations associated with stents having conventional radiopaque markers, or plated markings, it would be desirable to employ radiopaque markers or markings that do not limit the expansion capabilities of an expandable stent, nor alter the profile of the stent, that are clearly visible, that provide means to assess stent length and diameter, that do not obscure the blood vessel lesion being repaired and that are not detrimentally affected by galvanic corrosion.
The present invention provides for the radiopaque marking of a stent that effectively identifies the position, diameter and length of a stent both while fitted to the delivery device as well as upon implantation within a blood vessel, yet does not obscure the lesion being repaired. The marking is an integral part of a stent, does not protrude from the surface of the stent and does limit the expansion capabilities of the stent. Furthermore, the marking is not adversely affected by galvanic corrosion. The radiopaque marking of the present invention may be adapted to stents having various geometric shapes and that are constructed of any of various materials.
In a preferred embodiment, the stent of the present invention is of wire construction wherein a plurality of shaped wire rings are axially aligned and appropriately linked. This type of stent is well known in the art and offers significant advantages in terms of expandability, radial strength, longitudinal flexibility, and longitudinal stability during expansion. However, no radiopaque materials are known that satisfy the strength and biocompatibility requirements of such application. In order to render such stents fluoroscopically visible in accordance with the present invention, the proximal most and distal most rings are wholly formed of a radiopaque material.
Such construction provides all of the advantages set forth above. Positioning of the radiopaque material at the extreme ends of the stent renders the position of the stent clearly and precisely ascertainable without interfering with the visibility of a lesion therebetween. Because the end rings are wholly constructed of radiopaque material, they create a brilliant image when fluoroscopically illuminated, unequivocally revealing the position of the stent. Additionally, because the structure of the two end rings is substantially identical to the structure of the stent""s other rings, all of the rings, including the end rings expand at the same rate during deployment of the stent. This causes the marker rings to engage the vessel walls in the same manner as the stent""s other rings and thus become firmly embedded therein. Should the end rings eventually become separated from the balance of the stent, there is no danger of them being carried downstream or otherwise create an obstruction due to their expanded configuration and growth of an internal layer over the stent. Additionally, because the end rings are of the same size and shape as the rings in the body of the stent, the radiopaque marking of the stent does not pose an impediment to the flow of blood thereover and is less likely therefore to lead to platelet activation and thrombosis formation. Similarly, there is no likelihood of the marker impinging into the vessel walls to an inordinate degree and cause trauma.
The stent may be constructed using any of a variety of techniques including fabricating the non-radiopaque series of linked rings and the radiopaque end rings separately and then subsequently joining them such as by laser welding. Alternatively, a tube may be constructed wherein a flat section of radio-transparent material is first joined to two flat strips of radiopaque material afterwhich the joined material is formed into a cylinder. Fabrication of the stent may be accomplished in the conventional manner by then laser cutting or etching the cylinder to define an expandable configuration of struts.
These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention.